Dietary composition of young sturgeons (Acipenseridae) from the Bulgarian section of the Danube River

All sturgeon species currently living in the Danube River are either critically endangered or at the edge of extinction. Sturgeons are mainly benthic feeders, with a diet of molluscs, crustaceans, and small fishes. During the last two decades, we have seen a significant change in the species composition of benthic invertebrates in the Danube River as a result of anthropogenic impacts - hydrological and habitat modifications, introduction of alien and invasive species. The aim of this study was to investigate the food composition and the importance of different prey taxa, and to describe the feeding strategies of young sturgeons along the Bulgarian section of the Danube River. From June to August 2019, the stomach contents of 61 fish specimens from the species Acipenser stellatus, A. ruthenus, A. gueldenstaedtii were collected using gastric lavage. It was found that Obesogammarus obesus (Amphipoda) prevails in the diet of young of the year (YOY) A. stellatus and A. gueldenstaedtii, while Corbicula fluminea (Bivalvia) was the main food item of A. ruthenus. Chironomidae larvae also had high occurrences in the diet of all sturgeon species. For the YOY specimens up to 35 cm total length, nearly 94% of prey items belonged to O. obesus and diet overlap was observed. Improved knowledge about the diet of the young sturgeons could lead to the determination of the species' major feeding habitats in the Danube River, providing crucial information for their protection and restoration.